Reduction of friction and resistance in the substrate surface or interface between substrates has currently been an important subject in a number of different technical areas. Lubricants such as oil and grease have been used to reduce friction on hard surfaces such as metal and the like.
Powder lubricants such as boron nitride, nylon powder, molybdenum compound and N-lauroyllysine are also used for the treatment of fibers and hard surfaces. Since these compounds provide lubricity to the skin and hair, they are also used for cosmetics.
Since N-lauroyllysine has a particular plate-like crystal structure, and good slidability resulting therefrom, it is used as a powder material in the fields of cosmetic, lubricant and the like. However, since N-lauroyllysine has high water repellency and is poor in compatibility with water, it cannot be blended with an aqueous composition with ease and addition thereof at a high concentration is not available. Moreover, since N-lauroyllysin agglomerates in the obtained aqueous composition, the composition problematically lacks smooth texture.
Patent document 1 describes an embodiment wherein N-mono long-chain acyl basic amino acid crystals are appropriately pulverized, and used as a pearlizer in shampoo. However, it is still difficult to contain N-mono long-chain acyl basic amino acid stably at a concentration of not less than 5%, and the aforementioned problems remain unsolved.
On the other hand, patent document 2 proposes a method of neutralization crystallization of N-mono long-chain acyl basic amino acid to give ultrafine crystals to be added to cosmetics. As a method for obtaining crystals by neutralization-crystallization, patent document 3 describes a method including adding crude crystals of N-lauroyllysine to an aqueous medium at pH 11 or above, heating the mixture with stirring to 40 to 60° C. to dissolve the crystals, and neutralizing the mixture to pH 6.5 by dropwise addition of hydrochloric acid to allow crystallization. Patent document 4 describes a method of controlling the crystal diameter of Nε-long chain acyllysine by changing the crystallization conditions.
However, all these techniques aim to obtain crystal with a particular size and a particular shape in the form of a powder, rather than solve the problems of N-lauroyllysine such as poor compatibility with water and agglomeration and settling in a dispersion of N-lauroyllysine. Therefore, it is still difficult to contain N-lauroyllysine at a high concentration in cosmetics, and the problem of agglomeration in the composition remains unsolved.